Method for heat-treating work pieces made of temperature-resistant steels

ABSTRACT

A method of producing a workpiece of a heat-resistant steel, in particular hot forming tool steel, is described, the workpiece being hardened and depassivated after mechanical machining and electrochemical treatment, the hardening including a reduction step, so that no depassivation need be performed by pickling, for example, before nitriding, and the result of the hardening treatment is a favorable surface condition for stepwise nitriding.

[0001] The present patent application relates to a method for heat treatment of a workpiece made of heat-resistant steel, in particular hot forming tool steel, the workpiece being hardened and nitrided after mechanical working and electrochemical treatment, reduction of the workpiece surface being performed during hardening without having to perform a pickling treatment before the subsequent nitriding.

[0002] Nozzle bodies for modern direct injection systems are used to an increasing extent at operating temperatures up to 450° C. High demands are therefore made on the strength of components and the wear resistance of nozzle bodies. Nitrided hot forming tool steel in particular is therefore used to manufacture the nozzle bodies. ECM (electrochemical machining) methods are used in the production of internal bores (pressure chambers) and for rounding. The ECM methods used for shaping and surface treatment of metal workpieces are performed in an electrolyte solution, the workpiece to be machined usually being connected as the anode and the tool being connected as the cathode. Electrochemical machining methods are used in particular for deburring, polishing, grinding and etching the surfaces of a workpiece. The workpieces formed by the ECM method are highly passive and are very difficult to treat by thermochemical diffusion methods, in particular nitriding, because more noble alloy elements such as Cr remain on the surface and/or oxide alloy elements become oxidized, forming metal oxides and metal hydroxides Me_(x)O_(y)[OH]_(z).

[0003] To improve the nitridability of direct-injection nozzle bodies, it is conventional today to pickle passive surfaces before nitriding, in particular by using hydrochloric acid. However, pickling has some major disadvantages. Pickling with acid may cause pickling scars, which decrease the strength of the component. Furthermore, it is very difficult to reproduce the results of pickling, because the length of storage between machining, basic heat treatment and nitriding may vary. Furthermore, pickling results in a considerable additional cost which is attributable in particular to the cost of the installation used for pickling and the required labor cost. Pickled workpieces must also be cleaned after pickling by using a very complex special cleaning technique. Disposal of pickling solutions is also complicated. In addition, pickling with acid results in unwanted environmental pollution and has a negative effect on working conditions.

[0004] The object of the present invention is thus to develop a method of treating workpieces made of hot forming tool steel, in particular direct injection nozzle bodies, to improve the nitridability of these workpieces in particular without having to pickle the workpieces and to thus avoid the disadvantages due to pickling which are known in the related art.

[0005] The present invention achieves this object by providing a method of producing a workpiece of a heat-resistant steel, in particular a hot forming tool steel, the workpiece being hardened and thereby depassivated, characterized in that the hardening step includes a reduction treatment, in particular by using hydrogen, and then according to the present invention, the tempered workpieces having the active surface are nitrided in several steps under different gas atmospheres, the nitriding being performed first in an atmosphere of ammonia and an oxidizing agent, in particular water vapor or air, and then in an atmosphere of ammonia and a carbonaceous gas, in particular endogas or a mixture containing CO and/or CO₂.

[0006] The advantages of the method according to the present invention for heat treatment and of heat-resistant workpieces produced in this way from hot forming tool steel, in particular direct-injection nozzle bodies, are the result in particular of eliminating the pickling treatment before nitriding. Since no pickling is performed according to the present invention, no pickling scars are formed on the surface of the workpiece. Therefore, workpieces produced in this way have very advantageous strength properties. Since the method according to the present invention greatly improves the nitridability of the workpiece surfaces, the workpieces are also characterized by extremely uniform entire internal and external nitride layers. The method according to the present invention is also much less expensive in comparison with the method known in the related art because the installations required for pickling and subsequent cleaning are eliminated, and only equipment for supplying hydrogen to the vacuum hardening installation is needed. Since no acids are used for pickling in the method according to the present invention, this definitely results in less environmental pollution, and in particular it also improves working conditions.

[0007] Therefore, according to the present invention, the workpiece made of a heat-resistant steel, in particular a hot forming tool steel, is hardened and thereby depassivated, and the hardening step includes a reduction treatment. This reduction causes metal oxide layers and/or metal hydroxide layers on the surface of the workpiece to be removed, so that the subsequent nitriding is greatly improved without having to perform pickling. It is especially preferable according to the present invention for the reduction treatment to be performed by using hydrogen.

[0008] In conjunction with the present invention, a hot forming tool steel is understood to be a steel which is constantly exposed to an elevated temperature during its use, in particular a temperature of more than 200° C. There must not be any structural changes in hot forming tool steel during use, but instead the structure must be sufficiently stable and must have good tempering properties. Hot forming tool steel must have different properties depending on the desired application. Important desired properties include in particular strength and hardness, which in turn determine wear resistance.

[0009] Hot forming tool steel must meet some special requirements with regard to use properties, including hot strength, which is achieved in particular by molybdenum, tungsten and fine-grained vanadium, good tempering properties, which are achieved by chromium, which together with molybdenum, nickel and manganese increases hardenability, and hot wear resistance, which is determined by the heat strength of the matrix and by the type and amount of special carbides. Direct-injection nozzle bodies of hot forming tool steel must have a very high wear resistance, for example.

[0010] In a preferred embodiment of the present invention, the workpiece made of a heat-resistant steel, in particular hot forming tool steel, may be mechanically machined and subjected to an electrochemical machining before hardening, i.e., to an ECM method which is performed in an electrolyte solution for shaping and surface treatment. Such a method may be used in particular for deburring, polishing, grinding and/or etching the workpiece. For example, internal bores may be produced by using an ECM method and rounding subsequently.

[0011] According to the present invention, the workpiece is subjected to cleaning in an aqueous cleaning medium, in particular a neutral cleaning agent, after the ECM method. The cleaning step according to the present invention prevents the development of thick layers of Me_(x)O_(y)[OH]_(z) on the surface of the workpiece. Following the cleaning step, the workpiece is dried. Next the workpiece may be hardened immediately. In one embodiment of the present invention, the workpiece is first preserved by suitable methods if it is to be stored for a prolonged period of time after the ECM machine; then after storage, immediately before hardening, it is cleaned again in a liquid cleaning medium.

[0012] According to the present invention, hardening which results in a change in structure of the hot forming tool steel as described above is performed in a single-chamber or multichamber vacuum furnace. Hardening includes convective heating of the workpiece under nitrogen. Convective heating of the workpiece is preferably performed under a nitrogen pressure greater than 0.8 bar. In another embodiment of the present invention, the workpiece may also be heated in vacuo. According to the present invention, the workpiece is heated at least up to the hardening temperature of the hot forming tool steel. The hardening temperature of hot forming tool steel is approximately 1040° C.

[0013] According to the present invention, after reaching a desired temperature, the nitrogen atmosphere or the vacuum is replaced by hydrogen. The hydrogen thus introduced acts as a reducing agent for reduction of the layers of metal oxide and/or metal hydroxide present on the tool surface and is introduced at a temperature of at least 400° C. according to the present invention. However, the temperatures at which hydrogen is introduced are preferably in the range of the hardening temperature. According to the present invention, the hydrogen partial pressure is approximately 1 to 100 mbar. The flow rate of the hydrogen feed is preferably 100 to 2000 L[STP]/h. Austenitization is preferably performed over a period of 10 to 40 minutes.

[0014] In an especially preferred embodiment of the present invention, the gas exchange is performed as a pulsating operation over a period of one to ten minutes. In other words, the hydrogen partial pressure is increased in a pulsating manner over a period of one to ten minutes in exchange with vacuum. This yields a better gas exchange according to the present invention, in particular with workpieces having blind boreholes.

[0015] According to the present invention, the hydrogen is pumped out before the end of austenitization to prevent the gas used for quenching in the following step from becoming contaminated with hydrogen.

[0016] According to the present invention, the austenitized workpiece is quenched in nitrogen at a pressure of 1 to 10 bar after holding it at the hardening temperature.

[0017] According to the present invention after hardening, in particular after quenching, the workpiece is subjected to at least one tempering step.

[0018] According to the present invention, the workpiece is tempered at a temperature of up to 650° C., the tempering of the workpieces taking place either in a nitrogen atmosphere or under a nitrogen-hydrogen atmosphere. When a nitrogen-hydrogen atmosphere is used, it contains up to 5% hydrogen. According to the present invention, tempering of the workpiece is performed in a vacuum furnace or an evacuable tempering furnace. The tempering step according to the present invention is performed for approximately one to two hours.

[0019] According to the present invention, there is the possibility of the workpiece being subjected to multiple tempering steps instead of just one. In an especially preferred embodiment, the workpiece is subjected to a first tempering step which lasts approximately one to two hours, during which it is heated to a temperature of 520° C., and following that it is subjected to a second tempering step, which also lasts approximately one to two hours and during which it is heated to a temperature of 610° C.

[0020] According to the present invention, the workpiece is nitrided after tempering. Nitriding results in hardening of the hot forming tool steel of which the workpiece is made. This is based on diffusion of nitrogen into the steel. This results in an incorporation of nitrogen at interlattice sites and formation of nitrides and addition of nitrogen onto carbides to form carbonitrides. Nitriding results in hard boundary areas, thus increasing the hardness, wear resistance and durability of the hot forming tool steel.

[0021] According to the present invention, the workpiece is transferred to a nitriding furnace immediately after hardening and tempering. The nitriding furnace used according to the present invention is preferably a purged chamber furnace or an evacuable retort oven.

[0022] In an especially preferred embodiment of the present invention, the workpieces in the nitriding furnace are heated from room temperature to a temperature of approximately 400° C. in a first step. Heating of the workpieces in the nitriding furnace is preferably performed in an ammonia atmosphere. Then in a second step the workpiece is heated up to the nitriding temperature, which is approximately between 500° C. and 600° C. Nitriding of the workpieces, which is performed following heating, includes the following steps according to the present invention:

[0023] step 1: nitriding in an atmosphere of ammonia and an oxidizing agent,

[0024] step 2: nitriding in an atmosphere of ammonia and a carbonaceous substance,

[0025] step 3: nitriding in an atmosphere of ammonia or a gas additive to reduce the nitriding index.

[0026] In other words, the workpiece is nitrided in a gas atmosphere which is changed incrementally. The oxidizing agent in step 1 is preferably 0.5 to 10 vol % water vapor or up to 15% air. The carbonaceous substance used in step 2 is preferably 1 to 10 vol % endogas. Endogas is obtained by endothermic reaction of hydrocarbons such as propane and is a mixture of 23.7 vol % CO, 31.5 vol % H₂ and 44.8 vol % N₂. In another preferred embodiment, CO and/or CO₂ may also be used in equivalent amounts as the carbonaceous substance. The nitriding in step 2 is referred to as gas oxycarburation and lasts more than four hours according to the present invention, preferably approximately 10 to 60 hours. After the gas oxycarburation reaction, which lasts more than four hours according to the present invention, a uniform nitride layer has already developed on the surface of the workpiece. Following step 2, i.e., in step 3, a treatment is performed according to the present invention in ammonia or by adding gas to reduce the nitriding index in order to reduce the growth of connecting layers.

[0027] The gas flow rate during nitriding depends on the effective furnace volume, preferably amounting to three times the effective furnace volume in L[STP]/h.

[0028] According to the present invention, the workpieces are cooled by using nitrogen after nitriding. The workpiece produced and treated by using the method according to the present invention may then be hard machined by conventional methods.

[0029] The method according to the present invention may be used in particular to produce heat-resistant direct-injection nozzle bodies of hot forming tool steel, the nozzle body being made of high-strength heat-resistant hot forming tool steel, in particular steel brands X40CrMoV51 and X38CrMoV51. The pressure chamber is machined further, and a manufacturing cycle which includes soft machining, ECM machining and subsequent directly linked cleaning in an aqueous cleaning medium, but no pickling treatment, is performed according to the present invention. Then the direct-injection nozzle bodies are hardened in a vacuum furnace in the temperature range between 1000° C. and 1070° C. under a pulsed hydrogen partial pressure of 1 to 100 mbar and next quenched in a stream of nitrogen gas at a pressure of 1 to 10 bar. Tempering is performed at a temperature of up to 650° C. in a nitrogen atmosphere or a nitrogen-hydrogen atmosphere. Subsequent nitriding is preferably performed at 510° C. to 590° C. over a period of 10 to 60 hours using the gas oxynitrocarburation method described above in a chamber furnace or an evacuable chamber furnace. Heat-resistant direct-injection nozzles bodies treated in this way have more advantageous strength properties because the nitride layer is uniformly developed and there are no pickling scars like those described in the related art.

[0030] Other advantageous embodiments of the present invention are derived from the subclaims. 

What is claimed is:
 1. A method for producing a workpiece from a heat-resistant steel, in particular from hot forming tool steel, the workpiece being hardened and nitrided, wherein the hardening step includes a reduction treatment, and in this context a depassivated surface is formed for stepwise nitriding.
 2. The method as recited in claim 1, wherein hydrogen is used as the reducing agent.
 3. The method as recited in claim 1 or 2, wherein the workpiece is machined and treated electrochemically before the hardening step.
 4. The method as recited in one of the preceding claims, wherein the workpiece is cleaned before the hardening step.
 5. The method as recited in claim 4, wherein the workpiece is cleaned in an aqueous cleaning medium.
 6. The method as recited in one of the preceding claims, wherein the workpiece is dried after cleaning.
 7. The method as recited in one of the preceding claims, wherein the hardening step includes convective heating of the workpiece under a nitrogen atmosphere or in vacuo.
 8. The method as recited in claim 7, wherein convective heating is performed under a nitrogen pressure greater than 0.8 bar.
 9. The method as recited in claim 7 or 8, wherein the workpiece is heated at least to the hardening temperature of the hot forming tool steel.
 10. The method as recited in one of claims 7 through 9, wherein after reaching the desired temperature, the nitrogen atmosphere or the vacuum is replaced by a hydrogen atmosphere.
 11. The method as recited in claim 10, wherein the hydrogen atmosphere is generated in a pulsating operation over a pulse period of one to ten minutes.
 12. The method as recited in claim 10 or 11, wherein the hydrogen partial pressure is 1 to 100 mbar.
 13. The method as recited in one of claims 10 through 12, wherein the hydrogen flow rate is 100 to 2000 L[STP]/h.
 14. The method as recited in one of claims 7 through 13, wherein the hardening step is performed in a single-chamber or multichamber vacuum furnace.
 15. The method as recited in one of claims 7 through 14, wherein the workpiece is quenched after hardening.
 16. The method as recited in claim 15, wherein the workpiece is quenched using nitrogen.
 17. The method as recited in claim 15 or 16, wherein the nitrogen has a pressure of 1 to 10 bar.
 18. The method as recited in one of the preceding claims, wherein a tempering step is performed after hardening.
 19. The method as recited in claim 18, wherein the tempering step includes heating the workpiece up to a temperature of 650° C.
 20. The method as recited in claim 18 or 19, wherein the workpiece is heated in a nitrogen atmosphere.
 21. The method as recited in claim 18 or 19, wherein the workpiece is heated in a nitrogen-hydrogen atmosphere having a hydrogen content of up to 5%.
 22. The method as recited in one of claims 18 through 21, wherein tempering is performed in a vacuum furnace or in an evacuable tempering furnace.
 23. The method as recited in one of claims 18 through 22, wherein tempering is performed over a period of 1 to 4 hours.
 24. The method as recited in one of the preceding claims, wherein the workpiece is treated by nitriding.
 25. The method as recited in claim 24, wherein in a first step, the workpiece is heated from room temperature up to a temperature of approximately 400° C.
 26. The method as recited in claim 25, wherein the workpiece is heated under an ammonia atmosphere.
 27. The method as recited in one of claims 24 through 26, wherein the workpiece is heated up to the nitriding temperature.
 28. The method as recited in one of claims 24 through 27, wherein nitriding of the workpiece includes the following steps: step 1: nitriding under an atmosphere of ammonia and an oxidizing agent, step 2: nitriding under an atmosphere of ammonia and a carbonaceous substance, step 3: nitriding under an atmosphere of ammonia or a gas additive to reduce the nitriding index.
 29. The method as recited in claim 28, wherein 0.5 to 10 vol % water vapor or up to 15% air is used as the oxidizing agent.
 30. The method as recited in claim 28 or 29, wherein 1 to 10 vol % endogas or CO and CO₂ in equivalent amounts is used as the carbonaceous substance.
 31. The method as recited in one of the preceding claims, wherein after nitriding the workpiece is cooled under nitrogen.
 32. The method as recited in one of the preceding claims, wherein after cooling, the workpiece is hard machined.
 33. The method as recited in one of the preceding claims, wherein the workpiece is a direct-injection nozzle body.
 34. A direct-injection nozzle body, wherein it is produced by using a method as recited in one of claims 1 through
 33. 